ts mutants of influenza A virus were produced and assigned to 8 recombination groups. Host dependent ts mutation was observed and found to be widely distributed through the viral genome. Intracistronic complementation was also observed for ts mutations affecting the P1, P2, NA and NP genes. The mutations in the ts-1A2 master donor virus were shown to affect the genes coding for the polymerase 1 (P1) and polymerase 3 (P3) proteins. Genotype analysis indicated that transfer of the P1 and P3 ts genes from the ts-1A2 donor to 11 distinct virulent, wild type viruses led to a defined and reproducible level of attenuation of these recombinants for the hamster's respiratory tract. ts-1A2 recombinants were stable genetically during growth in experimental animals. However, one doubly seronegative child shed ts ion virus late in the course of infection with a ts 1A2 recombinant. In this instance shift from the ts to ts ion phenotype was shown to be associated with an extragenic suppressor mutation.